Electroless or autocatalytic plating of dielectric substrates finds wide-spread utility in the preparation of such diverse articles as printed circuits, automotive trim, etc.
In the normal commercial electroless plating process, the dielectric substrate, which has been preferably etched by physical or chemical means to improve metal adhesion, is sensitized by exposure of a solution of stannous ions, e.g., stannous chloride solution, and then activated by exposure to a solution of palladium ions, e.g., a palladium chloride solution. This activation is effected by reduction of the palladium ions to the zero valence state by the stannous ions to form palladium metal sites or by the formation of a tin/palladium complex on the surface of the dielectric substrate.
Thereafter, the activated substrate is plated by exposure to an electroless plating bath containing ions of the metal to be plated and a reducing agent capable of reducing (heterogenously) the valence state of the plating ions present in bulk solution to the metallic state. In conventional processes, copper is plated using an electroless plating bath comprised of copper ions and formaldehyde as a reducing agent. In the plating of nickel or cobalt, the reducing agent commonly used is sodium hypophosphite.
More recently, processes have been developed for electroless plating without the necessity of using palladium or other precious metals. For example, U.S. Pat. Nos. 3,772,056 and 3,772,078, both issued Nov. 13, 1973, to Polichette et al. describe processes in which previously-etched, non-metalic substrates are coated with a solution containing non-precious metal ions, i.e., ions of copper, nickel, cobalt or iron, and dried to form an adherent coating of the metal ions. Thereafter, the metal ions are reduced to the metallic state and the substrate is plated with a compatible electroless plating bath.
U.S. patent application Ser. No. 521,901, filed Nov. 8, 1974, as a continuation-in-part of U.S. Ser. No. 422,774, filed Dec. 7, 1973, by the present applicant, describes another procedure for effecting electroless plating of non-metallic substrates without the necessity of using palladium or other precious metal ions. In the processes described therein, a non-metallic substrate is contacted with a stannous and copper ions to form a stannous-cuprous complex on the surface of the substrate. The copper ions are then reduced to their metallic state using a suitable reducing agent.
Still another procedure is described in U.S. patent application Ser. No. 512,224, filed Oct. 4, 1974, by the present applicant. In the process described therein, hydrous oxide colloids of metal ions are coated on the surface of a non-metallic substrate. The substrate is then rinsed and immersed, without the necessity of prior drying step, in a solution containing a reducing agent capable of reducing the metal ions to the metallic state.
While significant cost savings are realized by coating of the substrates with non-precious metal ions, as exemplified by the above disclosures, instead of with the more expensive palladium or other precious metal ions, care must be exercised in the selection of electroless plating bath used with such systems. Specifically, conventional hypophosphite baths are not effective in the plating of nickel or cobalt onto the surface of substrates prepared using non-precious metals, e.g., copper, in a commercially suitable manner. Instead, it is necessary in the plating of nickel and cobalt to use an electroless plating bath containing a stronger reducing agent such as a boron reducing agent, e.g., an amine-borane, such baths being disclosed, for example, in U.S. Pat. No. 3,338,726, or a borohydride, as shown in U.S. Pat. Nos. 2,461,661 and 3,045,334. Such reducing agents, because of their relatively higher cost, diminish the commerical savings to be realized in the use of such procedures. Also, in useing the preceding non-precious metal systems, a lower site density is realized in some instances, thus reducing the speed and effectiveness of plating onto the prepared substates.
Procedures permitting the utilization of non-precious metal activated substrates while eliminating or minimizing the aforesaid disadvantages and permitting the utilization of conventional, commercially available electroless plating baths would be highly desirable.